1. Field of the Invention
The present invention pertains to an image forming apparatus, such as a copying machine, printer, or like device, and a method for operating such an image forming apparatus.
2. Description of the Related Art
An image forming apparatus using an electrophotographic image forming technique, as used, for example, in a digital copying machine, can determine characteristics of an input image and then perform processing which is suited to the determined characteristics. For example, such an apparatus can emphasize the edges of characters in images containing character data. To perform this function, an edge is recognized in the image data based on whether or not the difference in density between a target pixel and its surrounding pixels exceeds a prescribed threshold value. A latent image is then formed on a photosensitive unit based on the image data thus processed. The latent image is developed, and then the developed image is transferred to a sheet of paper. In this manner, the image of a character in which the edges are emphasized can be reproduced.
In a conventional image forming apparatus, when the photosensitive unit is exposed to light in order to form a latent image, the electric charge (comprising electrons and electron holes) occurring at exposed areas of the photosensitive unit becomes diffused as it moves toward the surface of the photosensitive unit. As a result, the electric charge can expand to these exposed areas, which leads to thickening of the image.
In an image forming apparatus using an organic photosensitive unit, the film thickness disposed on the photosensitive unit varies depending on how often the photosensitive unit has been used. The film thickness, in turn, affects the level of diffusion of the electric charge. When the film is thick, the range of diffusion is relatively large, and when the film is thin, the range of diffusion is relatively small. Therefore, the latent image formed by such an apparatus changes depending on how often the photosensitive unit has been used. This effect is particularly noticeable in line images, in which the thickness of the line image changes as a function of the amount of use of the photosensitive unit. Generally, the line image increases in thickness as the film increases in thickness.
A problem therefore arises in conventional reverse image forming apparatuses in that the thickness of a line image varies depending on how often the photosensitive unit is used. In other words, the conventional apparatuses do not provide consistent line image thicknesses over time.